Conventional resistance welding electrodes have a nose with a welding face formed on the distal end of the nose and a generally truncated cone surface formed on the outer surface of the nose. Such welding noses required periodic dressing to remove wing formations, pitting and other malformations formed on the outer surface during the welding process.
Welding electrode designs include a nose of copper formed of either copper-chromium or copper-chromium-zinc and any of the low alloy material variants of such copper material.
One problem with such prior cooper welding noses arises when the noses are used to resistance weld material having different thermal expansion properties than that of the base material of the welding nose or nose.
One example of such a welding process is in the resistance welding of galvanized steel where spaced electrodes are positioned on either side of workpieces and are moved to pressure a welding nose against the workpieces. Electrical current is pulsed through the electrodes heating the workpieces to spot weld the workpieces by forming a weld nugget therebetween. In such cases, the electrode nose dissipates heat from the work material by radiation and through conductive cooling at a water cooled shank. The nose has a cross-section to provide adequate strength to reduce metal fatigue.
In the past, smooth, cone shaped welding noses had a limited surface area for heat dissipation by radiation. Additionally, the reduced cross-sectional area and conically configured surface of such prior art noses could become fatigued.
Another characteristic of such prior art noses is that they are formed from extruded copper alloy bars. The extruded alloy bars have a metal structure in which the grains of the copper alloy material are arranged normal to the outer welding surface of the nose. There is a tendency for the metal grains in such nose designs to flow toward a void, e.g., the outer surface of the welding nose between the welding face and the base of the nose. Such grain flow creates wings of material at the O.D. of the nose that fracture when subject to differential thermal expansion between the material of the workpiece and the copper alloy material of the welding nose. The differential thermal expansion problem is especially noticeable when the copper alloy welding noses are used to resistance weld galvanized steel. In such cases, the zinc in the galvanizing material will alloy with the copper material of the welding nose. The wings contain sub-surface fractures due to either thermal shock produced by the large differences in the thermal expansion rates of copper alloys and the coated galvanized material or to alloy boundaries which are produced by migration of the zinc of the galvanized coating into the outer surface of the copper welding nose.
Another problem with such prior art copper spot welding noses is that pitting of the outer welding surface on the welding nose is caused by the coated material alloying with the copper in the welding nose or electrode. Such points of alloying will pull away from the electrode when the concentration of the coating material in the copper base material of the welding electrode becomes too great so as to produce surface pits. In the past, manufacturers have hardened the base material of the copper alloy welding electrode to prevent such pitting. Hardening, however, causes the electrode to become brittle with resultant fracturing when the electrode is subject to impact forces commonly produced as the electrode noses are pressed against the workpieces during a resistance spot welding process. Brittle material fractures can result. Such fracturing causes an increase in the growth rate of pitting of the welding surface of prior art spot welding noses.
Such sub-surface fracturing and surface pitting require continual dressing of the outer surface of the electrode's welding surface in order to maintain a specified mean button diameter (MBD) of the weld nugget. Such redressing interrupts the production welding cycle causing down time during the total life of the welding electrode.